Hundreds of millions of people suffer from chronic respiratory diseases. According to the latest World Health Organisation (WHO) estimates (2011), currently 235 million people have asthma, 64 million people have chronic obstructive pulmonary disease (COPD) and millions have other often-underdiagnosed chronic respiratory diseases.
Accurately measuring breathing capacity is important in the management of asthma and other respiratory conditions and in particular in predicting (and controlling) asthma attacks in susceptible individuals. During an acute asthma attack, the muscles of the upper airways contract, resulting in the partial or complete obstruction of the airways and making it harder for the lungs to take in and release air. However, narrowing of the airways is not confined to the onset of the attack, but rather builds up gradually over time. Often bronchial inflammation, causing a narrowing the airways, may have begun some time before the first symptoms of asthma are felt by the individual. A range of effective anti-asthma drugs are available which can substantially limit or eliminate such attacks, but these must be administered appropriately to avoid negative effects associated with inappropriate dosage.
The chronic nature of asthma therefore necessitates regular monitoring of respiratory function in susceptible individuals to detect symptoms which are prognostic of bronchial inflammation as early as possible and for practical reasons typically involves a combination of self-assessment and periodic assessment by a clinician.
It is well established that measurement of peak expiratory flow rate (PEF) usually measured in litres per minute, which indicates the speed with which air is blown out of the lungs, provides a reliable indication of respiratory function (Global strategy for asthma management and prevention. Bethesda (Md.): Global Initiative for Asthma, 2012). Simple mechanical devices are known for this purpose such as Peak Flow Meters (PFMs). These devices are simply constructed and typically consist of a plastic tube with a mouthpiece on one end.
In use, when a patient exhales into the mouthpiece of the tube, the force of the expiratory flow causes an opposing and reciprocating plate and an externally visible marker or pointer, often located in a channel or groove within the tube, to be impelled from a resettable start position (or zero) to a position corresponding to the maximum, or peak, expiratory flow of a single exhalation. Commonly, the PFM will incorporate a calibrated scale adjacent to the marker so that the individual can visualize and manually record the peak expiratory flow of that exhalation as indicated by the distance traveled by the marker along the scale. Repeated measurement of this kind is made in order to monitor changes in lung function, such as those that might be result from asthma or other respiratory ailments.
Some examples of PFMs incorporate an elastic device, such as a spring, to return the plate, moveably arranged within the tube, to the starting position as the force of the expiratory flow declines from a maximum value and becomes unequal to the elastic force of the spring, the opposing force of the spring returns the moveable plate towards its starting position. The marker, however, not being connected to the movable plate, remains at the location of maximum movement from the starting position, indicating the maximal distance moved by the plate during that expiration, allowing peak expiratory flow to be calculated.
PFMs which are currently available have a number of limitations. Firstly, the utility of PFMs is limited by their size, which makes inconspicuous transport or use difficult. As a result, the ability to monitor respiratory function at the required times can compromised.
Secondly, PFM devices generally require the results of tests to be taken, recorded and interpreted manually by the user at periodic intervals which leads to inaccuracies in the recordation of breathing characteristics and a failure to record results consistently over an extended period. This can adversely affect the diagnosis of individual risk factors which is a fundamental consideration in determining appropriate treatment regimens for a patient.
Thirdly, the process of taking and recording the readings can be time-consuming, which in combination with often cumbersome equipment required for analysis, frequently results in a disinclination on the part of the patient to make regular measurements of PEF. Poor adherence to the monitoring regimen, giving rise to sporadic datasets is a major problem in effective asthma management.
Fourthly, the PFM does not enable any more advanced monitoring of breathing function other than a measure of PEF.
More advanced devices are known, such as that marketed by MIR (http://www.spirometry.com/) in which a disposable turbine spirometer is used in conjunction with a proprietary monitoring system which optically monitors the rotation rate of the turbine when the passage of air causes it to rotate. However, these devices are expensive and bulky. The result is that the aforementioned system does not lend itself to use by individuals and as a consequence it is generally used by medical practitioners.
With the above in mind, the present invention has been devised. The invention seeks to improve the efficiency, accuracy, reliability and convenience of the self-assessment of respiratory function in susceptible individuals, particularly the monitoring of peak expiratory flow for the purposes of asthma management.